Centrifugal pump with mating case plate volute halves and constant section impeller

ABSTRACT

A centrifugal pump having front and back case plates with onehalf of a volute wall formed on each of the front and back case plates, the volute wall halves corresponding diametrically and in length such that when the case plates are disposed in abutting confronting relationship in an assembled pump the volute wall halves cooperate to form a complete volute passage. A radial force balancing partition is provided between an impeller confined by the case plates and the volute wall. The force balancing partition is similarly comprised of identical mating halves formed on the front and back case plates. The case plates house or confine an impeller having a constant wall thickness or cross-section throughout to substantially eliminate thermal shock in the impeller. The impeller includes recesses or cut-out areas in the opposite faces thereof to yield substantially greater hydraulic performance while at the same time reducing wear caused by abrasion.

United States Patent Zagar [151 3,656,061 [451 Apr. 10,1972

1541 CENTRIFUGAL PUMP WITH MATING CASE PLATE VOLUTE HALVES AND CONSTANTSECTION IMPELLER [72] Inventor: Irvin F. Zagar, Denver, C010. [73]Assignee: A. R. Wilfley and Sons, Inc., Denver,

[22] Filed: .Apr. 15,1970 [21] Appl.No.: 28,735

[52] U.S.Cl ..4l5/l09,415/197,415/214, 415/213 [51] Int.Cl...F04d29/02,F04d29/66,F04d 29/00 [58] Field of Search ..415/204,206,211, 109, 170 C, 415/196, 197, 200, 219, 34, 213

[56] References Cited UNITED STATES PATENTS 2,260,946 10/1941 Korte eta]..415/206 2,955,540 10/1960 Pawlicki ..415/204 2,571,802 10/1951 Wilfleyet a1 ..415/34 3,040,670 6/1962 Schenck et a1 ..415/169 3,043,229 7/1962Atwood, Jr. et a1. .....415/204 3,115,097 12/1963 Zagaret a1 ..415/1973,149,574 9/1964 Mill ..415/219 WWI 1oz 8 I E= l B i 11 is lllllVII/III! FOREIGN PATENTS OR APPLICATIONS Primary Examiner-Henry F.Raduazo AnomeyShoe'maker & Mattare 57 ABSTRACT A centrifugal pump havingfront and back case plates with one-half of a volute wall formed on eachof the front and back case plates, the volute wall halves correspondingdiametrically and in length such that when the case plates are disposedin abutting confronting relationship in an assembled pump the volutewall halves cooperate to form a complete volute passage. A radial forcebalancing partition is provided between an impeller confined by the caseplates and the volute wall. The force balancing partition is similarlycomprised of identical mating halves formed on the front and back caseplates. The case plates house or confine an impeller having a constantwall thickness or cross-section throughout to substantially eliminatethermal shock in the impeller. The impeller includes recesses or cut-outareas in the opposite faces thereof to yield substantially greaterhydraulic performance while at the same time reducing wear caused byabrasion.

19 Claims, 14 Drawing Figures 2 l Q "J E I a w lLll llim lllllllllli l 5PATENTEDAPRMBIQIZ 3,656,861 I SHEET 10F 7 INVENTOR. IRVIN E ZAGARATTORNEYS PATENTEDAPR 18 I972 SHEET 2 BF 7 INVENTOR. IRVIN E ZAGAR%.A/M1 77m ATTORNEYS PATENTEDAPR 18 I972 SHEET 3 [)F 7 INVENTOR IRVIN FZAGAR BY WM! 7 W ATTORNEYS PATENTEIJAPRIB QR 3,656,861

' SHEET NF 7 INVENTOR. Q IRVIN E ZAGAR ATTORNEYS PATENTEDAPR 18 1912SHEET 5 OF I (Hill Ml: I I l l lllll fi E H i INVENTOR. IRVIN E ZAGARJFJLgAD ATTORNEYS PATENTEUAPR 18 I972 SHEET 6 OF 7 INVENTOH. IRVIN E 2/1GA R (ya/Ml 77m ATTORNEYS PATENTEUAPR 18 I972 3,556,861 SHEET 7 BF 7INVENTOR. IRVIN E ZAGAR BY ATTORNEYS CENTRIIFUGAL PUMIP WITH MATWG CASEPLATE VOLUTE HALVES AND CONSTANT SECTION IMPELLER BACKGROUND OF THEINVENTION This invention relates to centrifugal pumps and, moreparticularly, to a novel impeller and volute means therefor.

Manufacture of prior art pump volutes for centrifugal pumps involvesquite high foundry costs due to the necessity of volute core manufactureand rejects caused by core shifting and other foundry losses due tocomplex casting manufacture.

Further, in materials not readily moldable with cores, such as in sometypes of plastic, as vinylidene fluoride and tetraethylene fluoride,severe sidewall reinforcement problems exist in the prior art voluteconfigurations.

Moreover, a serious problem in the design of pump impellers is to makean impeller from relatively inexpensive materials suitable for aparticular purpose which will not be subjected to severe thermal shockdue to the varying thicknesses incorporated in prior art pump impellerdesigns.

OBJECTS OF THE INVENTION It is an object of this invention to achieveconsiderable savings in pattern making costs by eliminating the corenecessary in conventional volute castings and to reduce foundry costs byeliminating volute core manufacture and rejects from core shifting, andother foundry losses due to complex casting manufacture.

Another object of this invention is to enable simple molds to be used inthe production of volute configurations in materials not readilymoldable with cores, such as in some types of plastic, as vinylidenefluoride and tetraethylene fluoride, and to increase hydraulicperformance by eliminating compromises in conventional design of pumpvolutes and by providing additional features such as radial forcebalancing partitions in the case between the pump volute wall and theimpeller.

Another object of the invention is to make the case plate volute designeasily accessible for fabrication with wear resistant surfaces atspecific locations within the case volute, such as ceramic tiles andtungsten carbide tiles, and to obtain smoother volute surfaces becauseof the easily reached volute shape.

A further object of the invention is to provide an impeller having aconstant section or wall thickness throughout to render the impellersubstantially free from thermal shock by improving heat transfer in theimpeller to eliminate weakening of the impeller sections caused byexpansion and contraction of the impeller sections due to temperaturechanges in the urn p Aribther object is to considerably reduce theweight and cost of the impeller when expensive materials are used in itsconstruction and to substantially improve hydraulic performance andreduce wear caused by abrasion by providing cut-out areas oranti-recirculating vanes or ports on the front and rear shrouds of theimpeller.

A further object is to render the impeller capable of ready manufacturefrom hard-to-form materials and to reduce the cost of the impeller bysubstantially eliminating the number of rejects due to multiple cores,core shifting and other foundry costs due to complex castings.

Another object is to enable compromises in impeller design relative tovane requirements, such as number, area, size, curvature and shape, withno loss in hydraulic efficiency.

Other object of this invention will become apparent from a study of thefollowing detailed description and of the drawings, wherein likereference numerals refer to like parts throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is an elevational view partly insection of a centrifugal pump according to the present invention.

FIG. 2 is a sectional plan view taken along the line 2-2 of FIG 1.

FIG. 3 is a sectional plan view taken along the line 3-3 of FIG. 1.

FIGS. 4, 5 and 6 are partial sectional views taken along the line A-A inFIG. 2 and showing the various joint configurations which may beutilized.

FIG. 7 is an exploded perspective view of the case plate volutes, caseshell, and impeller.

FIG. 8 is a front plan view of the impeller according to the presentinvention depicting the pumping ports or vanes.

FIG. 9 is a top plan or edge view of the impeller according to thepresent invention.

FIG. 10 is a rear planview of the impeller depicting the expeller portsor vanes.

FIG. 11 is a sectional view taken on the line 11-11 in FIG. 10.

FIG. 12 is a sectional view in elevation of a conventional slurryhandling impeller illustrating the large clearance between stationaryand rotating parts.

FIG. 13 is a view similar to FIG. 12 of a slurry handling impelleraccording to the present invention showing how the clearance between thestationary and rotating parts is substantially reduced.

FIG. 14 is a partial sectional view taken along the line l4- 14 in FIG.13.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION As shown inFIG. 1, a base 1 has a frame F secured thereto as by cap screws or thelike 2 and a case support bracket B is secured to the frame F by meansof cap screws or the like 3. A pump housing or case C is secured to theforward portion of the case bracket B by means of suitable fasteners,such as cap screws or the like 4, and comprises a back housing wall 5having a central shaft receiving opening 6 therethrough and a fronthousing wall 7 having a central intake opening 8 therethrough in axialalignment with the opening 6 through the back wall 5 and held in spacedrelation to the back wall by means of an outer metallic case shell 9interposed peripherally between the marginal edges of the front and backwalls and secured therein by means of the fasteners 4 through the casebracket and fasteners 10 through the front wall 7.

A pump shaft 8 extends forwardly through opening 6 into case C and hasan impeller I with a constant cross-section or wall thickness suitablysecured to the forward end thereof. A stationary shaft seal housing 111is secured by suitable fastening means to the rear face of back wall 5in coaxial alignment over opening 6. The stationary shaft seal housingincludes a stepped inner surface which cooperates with a correspondinglystepped outer surface on a shaft seal sleeve 12 to form a sealinglabyrinth between the pump shaft and case. Suitable sealing means 13 and14 are disposed on the forward end of the shaft seal sleeve and on thestationary seal housing, respectively, in cooperative association toprovide a mechanical seal between the pump shaft and case. Duringoperation of the pump, the shaft S is shifted axially to the left asviewed in FIG. 1 by suitable speed responsive weight actuator means (notshown) to disengage seal 13 from seal 14, thus preventing undue wear ofthe mechanical sealing parts when the pump is running. A suitabledownwardly directed case drain 15 is in the lower portion of stationaryseal housing 11 to drain material from the housing which leaks past theseal means.

A pair of confronting mating front and back case plates 16 and 17 isconfined within the back and front housing walls 5 and 7 in surroundingrelation to the impeller l. The back case plate 17 has one face 18thereof substantially flat and in flush abutting contact with the frontsurface of back wall 5. A rearwardly extending annular projection 15extends from the face 18 and is snugly received in opening 6 throughback wall 5. A central opening 20 is formed through the projection forsnug fitting receipt of the pump shaft S and has an axially notched orgrooved inner surface which cooperates with axial projections on theimpeller hub to form a labyrinth passage therewith, as more fullydescribed in US. Pat. No. 3,115,097. The front case plate 16 has asimilar flat front surface in flush fitting contact with the innersurface of front wall 7 and includes a cylindrical forwardly extendingsleeve or projection 21 received snugly in inlet opening 8.

An inner case shell or liner 22 is disposed between the outer case shell9 and the outer peripheries of the case plate sections 16 and 17, andhas its opposite edge portions nested in notches or recesses 23 and 24in the marginal edge portions of each case plate section 16 and 17,respectively. Annular gasket seals 25 and 26 are compressed between theperipheral marginal edges of the back and front housing walls and 7 andthe opposite edges of the outer case shell 9 and between the peripheralmarginal edges of the case plate sections 16 and 17 and the oppositeedges of the inner case shell 22, effecting seals therebetween. Adischarge opening 27 is formed from the case through the inner and outercase shells, as seen most clearly in FIGS. 2 and 3.

As seen most clearly in FIGS. 1, 2, 3, and 7, each case plate has avolute wall configuration thereon which is substantially the mirrorimage of the volute wall configuration on the other case plate.Additionally, each case plate has a balancing partition or wall thereoninterposed between a portion of the volute wall and the impeller tobalance radial forces generated by the impeller.

In FIG. 2, the expeller portion E of the Impeller I is shown and thevolute wall 28 and balancing partition 29 on the from case plate 16 canbe seen. The volute wall comprises a relatively short straight portion30 in alignment with the outlet opening 27 and substantially flush withone side thereof and long spiraling portion 31 extending from the innerend of the straight portion 30 around the impeller I on an increasingradius to a position adjacent the outlet 27 and disposed at theperiphery of the case plate in contacting relationship with the innersurface of the inner case shell or liner 22 through which the opening 27is formed. The balancing partition 29 includes a substantially straightportion 32 disposed or positioned in the center ofthe outlet 27 and acurved or spiraled portion 33 having a decreasing radius extending fromthe straight portion between the volute wall 28 and the impeller I to aposition on the side of the impeller substantially diametricallyopposite the outlet from the pump. The shape and size of the balancingpartition in relation to the volute wall is such that the output orpressure generated by the impeller is divided into a pair ofsubstantially equal flow paths by the balancing partition tosigniftcantly reduce the radial forces imposed on the impeller due tothe pressure generated at the outlet from the volute configuration.

As seen in FIG. 3, the rear case plate 17 includes a substantiallyidentical volute wall configuration 34 and balancing partition 35thereon in surrounding relation to the impeller I and disposed such thatthe volute wall and balancing partition on the rear case plate arecommensurate in length and diametrically equal to the volute wall andbalancing partition on the front case plate whereby when the case platesare joined in assembled relation the volute wall 34 on the rear caseplate is in mating abutting contact with the volute wall 28 on the frontcase plate and the balancing partition 35 on the rear case plate is inmating abutting contact with the balancing partition 29 on the frontcase plate to form a closed volute passage and a balancing partitiondividing the outlet from the volute passage into two substantially equalportions. More than one partition or volute wall may be incorporated ifdesired or the partition may be omitted. The dimensions of the volutewalls and of the partitions are determined by the particular performancedesired.

Several embodiments of sealing means between the abutting confrontingsurfaces of the case plate sections 16 and 17 are shown in FIGS. 4, 5and 6. In FIG. 4, a simple labyrinth passage seal means 36 is shown. InFIG. 5, a tongue and groove joint 37 is shown, and in FIG. 6, a simpleflat sealing gasket 38 is interposed between the confronting abuttingsurfaces of the volute walls and balancing partitions on the case platesections. Further, as shown in FIG. 6, the peripheral or volute portionof the case plate sections may be made separate from the centralportions of the case plate sections and sealed thereto by gaskets or thelike 39 and 40, thus making the volute replaceable without necessitatingreplacement of the entire case plate sections.

The impeller I according to the present invention is seen more clearlyin FIGS. 7 through 11 and comprises in its preferred form a one-pieceintegrally fonned unit including pumping or impeller vanes or ports Pand expeller vanes or ports E. The impeller is of the closed type andthe pumping and expeller ports are formed along continuous straightlines and are disposed such that their center line axes or center linesare non-radial and offset with respect to the axis of rotation of theimpeller. This construction is more fully described in US. Pat. No.3,115,097.

This impeller is particularly unique and is an improvement over theimpeller in US. Pat. No. 3,115,097 in that the pumping ports P andexpeller ports E are raised from the opposite sides of a generallydisc-shaped central or dividing wall 41 and are defined by walls 42 andwalls 43, respectively. The walls 42 and 43 defining the pumping andexpeller ports are substantially equal in thickness with each other andwith the central or dividing wall 41. This construction achieves aconstant wall thickness throughout the impeller substantiallyeliminating thermal shock or stresses in the impeller due to temperaturefluctuations in the pump which, in prior art impellers, causes unevenexpansion or contraction of impeller sections due to the difference inwall thicknesses throughout the impeller. Moreover, the recesses orareas A, between the pumping ports P and the recesses or areas A,between the expeller ports E act or serve as anti-recirculating cut-outareas. In other words, the raised portions defining the pumping ports Pand the expeller ports E act much in the nature of vanes on an open typeimpeller and a substantial increase in the hydraulic head generated bythe impeller is realized and additional suction pressure may be withheldwithout leakage. Moreover, the provision of the cut-out areas A, and A,on the front and rear shrouds of the impeller, or conversely, raisingthe pumping ports P and expeller ports E from opposite sides of dividingwall 41 enables the clearance between the impeller and the case platesto be decreased thus further contributing to the efficiency of the pump,while at the same time a substantial improvement in abrasion wear isrealized due to the expulsion of particles by the anti-recirculatingcut-outs A, and A from the surfaces normally given a large clearance toavoid the abrading action. This is illustrated in FIGS. 12 and 12 whichshow a conventional slurry handling runner or impeller I and a slurryhandling impeller I according to the present invention, respectively.

The impeller I shown in FIGS. 13 and 14 is a modification of theimpeller shown in FIGS. 7 through 11 and comprises a slurry handlingrunner or impeller I having only pumping ports P. Material is cut awayfrom the front and rear faces or shrouds of the impeller I forming cutout areas or anti-recirculating cut-outs A and A, which function in thesame manner as do areas A, and A in the impeller I shown in FIG. 7 toimprove both hydraulic performance and abrasion wear. The sections orwalls 44, 45 and 46 are all substantially equal in thickness and theimpeller I is substantially free from thermal shock and stresses just asin the previously described impeller The impeller of the presentinvention may be manufactured with facility of parts from hard-to-formmaterials, such as vinylidene fluoride and tetraethylene fluoride,Kynar, phenolics and the like. These materials are very expensive andthe invention, because of a considerable reduction in material used dueto the cut-out areas, also results in a substantial cost savings.

The impellerI need not be used with the case plate sections 16 and 17but either could be used with other types of impellers or case plates.

In the specific impeller described, there are four pumping ports andeight expeller ports. The pumping ports are much larger in cross-sectionthan the expeller ports and are shorter than the expeller ports in orderto achieve the desired hydraulic characteristics. Of course, the numberand size of the pumping and expeller ports could be varied dependingupon the characteristics desired.

As this invention may be embodied in several forms without departingfrom the spirit or essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, and since thescope of the invention is defined by the appended claims, all changesthat fall within the metes and bounds of the claims or that form theirfunctional as well as conjointly cooperative equivalents are thereforeintended to be embraced by those claims.

1. A centrifugal pump having a case including front and back walls, acentral rotatable shaft supported in said case, shaft seal means sealingsaid shaft with respect to said case, impeller means suitably fixed onsaid shaft, case plate means confined within said case, said case platemeans comprising front and rear confronting mating case plate sections,the confronting faces of the case plate sections each having axiallyprojecting volute wall means thereon extending toward one another, thevolute wall means on the front case plate coinciding diametrically andin length with the volute wall means on the rear case plate and inmating abutting contact therewith in an assembled pump, said volute wallmeans positioned in surrounding relation to said impeller to form acomplete volute in the pump, and a substantially cylindrical inner caseshell interposed between the peripheral edges of thecase plate sectionsto define a chamber therewith.

2. A centrifugal pump as in claim ll, wherein a radial force balancingpartition is on the confronting faces of said front and rear case platesections, the force balancing partition on the front case platecoinciding diametrically and in length with the force balancingpartition on the rear case plate, said force balancing partitioninterposed between at least a portion of said volute wall and saidimpeller to balance radial forces imposed on said impeller.

3. A centrifugal pump as in claim 2, wherein the front and rear caseplate sections are sealed with respect to one another by labyrinth meanson the abutting surfaces of the volute walls and force balancingpartitions.

4. A centrifugal pump as in claim 2, wherein the front and rear caseplate sections are sealed with respect to one another by tongue andgroove seal means on the abutting surfaces of the volute walls and forcebalancing partitions.

5. A centrifugal pump as in claim 2, wherein the front and rear caseplate sections are sealed with respect to one another by means of a flatsealing gasket interposed between the abutting mating surfaces of thevolute walls and force balancing partitions.

6. A centrifugal pump as in claim 1, wherein a substantially cylindricalouter case shell is interposed between the marginal peripheral edges ofthe front and back walls.

7. A centrifugal pump as in claim 6, wherein said substantiallycylindrical inner case shell is immediately radially inwardly of theouter case shell.

8. A centrifugal pump as in claim 7, wherein the outer case shell issealed to the front and back walls and the inner case shell is sealed tothe front and rear case plates by a pair of annular sealing gasketsinterposed therebetween.

9. A centrifugal pump as in claim 8, wherein an inlet to the pump caseis formed through the front wall and front case plate section and anoutlet from the pump case is formed through the inner and outer caseshells.

10. A centrifugal pump as in claim 7, wherein the front and rear caseplate sections, the inner case shell and the impeller are formed ofaplastic material.

11. A centrifugal pump having a case including front and back walls, acentral rotatable shaft supported in said case, shaft seal means sealingsaid shaft with respect to said case, and impeller means fixed to saidshaft within said case, said impeller means being formed of a materialsuch as vinylidene fluoride or the like which is nonnally subject tothermal shock when subjected to temperature change, a plurality ofclosed pumping ports on the front of the impeller extending fromadjacent the axis to the periphery and a plurality of closed expellerports on the back thereof extending from adjacent the axis to theperiphery, alternate cut-out areas on the front and back of saidimpeller between the pumping ports and between the expeller ports,respectively, whereby the walls of the impeller are of constantthickness between the front and rear cut-out areas and around thepumping and expeller ports in a plane normal to the impeller axis and ina plane radial to the impeller axis, respectively, so as to reducethermal shock in the impeller.

12. A centrifugal pump as in claim 11, wherein there are fewer pumpingports than expeller ports, said pumping ports larger in cross-sectionand shorter in length than said expeller ports.

13. A centrifugal pump as in claim 12, wherein said pumping ports andsaid expeller ports are straight and are non radial and have theircenter lines or axes offset relative to the axis of rotation of saidimpeller.

M. A centrifugal pump as in claim llll, wherein said cut-out areasdefine anti-recirculating ports contributing to the hydraulic efficiencyof the pump and reducing wear caused by abrasion by expelling abrasivematerial from between the stationary parts and the rotating parts.

15. A centrifugal pump as in claim lll, wherein the pumping ports andexpeller ports are raised on opposite sides of a center or dividingwalland define therebetween the cut-out areas on the front and back of theimpeller between the pumping ports and expeller ports, respectively,said cut-out areas defining anti-recirculating ports contributing to thehydraulic efficiency of the pump and reducing wear caused by abrasion byexpelling abrasive material from between the stationary parts and therotating parts.

116. A centrifugal pump as in claim 11, wherein said pump includes caseplate sections confined between said front and back walls in surroundingrelation to said impeller.

17. A centrifugal pump as in claim 16, wherein said case plate sectionsinclude front and back mating abutting volute wall sections, the frontface of the back case plate having at least one forwardly projectingvolute wall thereon and the back face of the front case plate having atleast one rearwardly projecting volute wall thereon in mating abuttingcontact with the volute wall on the back case plate, said volute wallscoinciding diametrically and in length to form a complete volute wall orpassage in an assembled pump.

18. A centrifugal pump having an outer case including a front wall, aback wall, and a peripheral outer case ring; a central rotatable shaftsupported in said case; shaft seal means sealing said shaft with respectto said case; an inner case including an annular, peripheral, inner casering, and a pair of opposite, end case plates fitted to the oppositesides of said inner peripheral case ring and defining a chambertherewithin; impeller means carried by the shaft in said chamber; volutemeans on said case plates and extending into and across said chamber inabutting engagement with one another to form a complete volute and in atleast partially surrounding relationship to said impeller; and forcebalancing partition means on said case plates extending into saidchamber in abutting engagement and in operative relationship with saidimpeller and said volute means to balance radial forces imposed on theimpeller generated by pressure at the outlet of said volute means.

19. An impeller for use in a centrifugal pump, said impeller formed of amaterial such as vinylidene fluoride or the like which is normallysubject to thermal shock when subjected to temperature change, aplurality of closed pumping ports on the front of the impeller extendingfrom adjacent the axis to the periphery thereof, a plurality of closedexpeller ports on the back of the impeller extending from adjacent theaxis to the periphery thereof, alternate cut-out areas on the front andback of said impeller between the pumping ports and between the expellerports, respectively, the walls of the impeller being of constantthickness between the front and back cut-out areas and around thepumping and expeller ports in a plane normal to the impeller axis and ina plane radial to the impeller axis, respectively, so as to reducethermal shock in the impeller.

1. A centrifugal pump having a case including front and back walls, acentral rotatable shaft supported in said case, shaft seal means sealingsaid shaft with respect to said case, impeller means suitably fixed onsaid shaft, case plate means confined within said case, said case platemeans comprising front and rear confronting mating case plate sections,the confronting faces of the case plate sections each having axiallyprojecting volute wall means thereon extending toward one another, thevolute wall means on the front case plate coinciding diametrically andin length with the volute wall means on the rear case plate and inmating abutting contact therewith in an assembled pump, said volute wallmeans positioned in surrounding relation to said impeller to form acomplete volute in the pump, and a substantially cylindrical inner caseshell interposed between the peripheral edges of the case plate sectionsto define a chamber therewith.
 2. A centrifugal pump as in claim 1,wherein a radial force balancing partition is on the confronting facesof said front and rear casE plate sections, the force balancingpartition on the front case plate coinciding diametrically and in lengthwith the force balancing partition on the rear case plate, said forcebalancing partition interposed between at least a portion of said volutewall and said impeller to balance radial forces imposed on saidimpeller.
 3. A centrifugal pump as in claim 2, wherein the front andrear case plate sections are sealed with respect to one another bylabyrinth means on the abutting surfaces of the volute walls and forcebalancing partitions.
 4. A centrifugal pump as in claim 2, wherein thefront and rear case plate sections are sealed with respect to oneanother by tongue and groove seal means on the abutting surfaces of thevolute walls and force balancing partitions.
 5. A centrifugal pump as inclaim 2, wherein the front and rear case plate sections are sealed withrespect to one another by means of a flat sealing gasket interposedbetween the abutting mating surfaces of the volute walls and forcebalancing partitions.
 6. A centrifugal pump as in claim 1, wherein asubstantially cylindrical outer case shell is interposed between themarginal peripheral edges of the front and back walls.
 7. A centrifugalpump as in claim 6, wherein said substantially cylindrical inner caseshell is immediately radially inwardly of the outer case shell.
 8. Acentrifugal pump as in claim 7, wherein the outer case shell is sealedto the front and back walls and the inner case shell is sealed to thefront and rear case plates by a pair of annular sealing gasketsinterposed therebetween.
 9. A centrifugal pump as in claim 8, wherein aninlet to the pump case is formed through the front wall and front caseplate section and an outlet from the pump case is formed through theinner and outer case shells.
 10. A centrifugal pump as in claim 7,wherein the front and rear case plate sections, the inner case shell andthe impeller are formed of a plastic material.
 11. A centrifugal pumphaving a case including front and back walls, a central rotatable shaftsupported in said case, shaft seal means sealing said shaft with respectto said case, and impeller means fixed to said shaft within said case,said impeller means being formed of a material such as vinylidenefluoride or the like which is normally subject to thermal shock whensubjected to temperature change, a plurality of closed pumping ports onthe front of the impeller extending from adjacent the axis to theperiphery and a plurality of closed expeller ports on the back thereofextending from adjacent the axis to the periphery, alternate cut-outareas on the front and back of said impeller between the pumping portsand between the expeller ports, respectively, whereby the walls of theimpeller are of constant thickness between the front and rear cut-outareas and around the pumping and expeller ports in a plane normal to theimpeller axis and in a plane radial to the impeller axis, respectively,so as to reduce thermal shock in the impeller.
 12. A centrifugal pump asin claim 11, wherein there are fewer pumping ports than expeller ports,said pumping ports larger in cross-section and shorter in length thansaid expeller ports.
 13. A centrifugal pump as in claim 12, wherein saidpumping ports and said expeller ports are straight and are non-radialand have their center lines or axes offset relative to the axis ofrotation of said impeller.
 14. A centrifugal pump as in claim 11,wherein said cut-out areas define anti-recirculating ports contributingto the hydraulic efficiency of the pump and reducing wear caused byabrasion by expelling abrasive material from between the stationaryparts and the rotating parts.
 15. A centrifugal pump as in claim 11,wherein the pumping ports and expeller ports are raised on oppositesides of a center or dividing wall and define therebetween the cut-outareas on the front and back of the impeller between the pumping portsand expeller ports, respectively, said cuT-out areas defininganti-recirculating ports contributing to the hydraulic efficiency of thepump and reducing wear caused by abrasion by expelling abrasive materialfrom between the stationary parts and the rotating parts.
 16. Acentrifugal pump as in claim 11, wherein said pump includes case platesections confined between said front and back walls in surroundingrelation to said impeller.
 17. A centrifugal pump as in claim 16,wherein said case plate sections include front and back mating abuttingvolute wall sections, the front face of the back case plate having atleast one forwardly projecting volute wall thereon and the back face ofthe front case plate having at least one rearwardly projecting volutewall thereon in mating abutting contact with the volute wall on the backcase plate, said volute walls coinciding diametrically and in length toform a complete volute wall or passage in an assembled pump.
 18. Acentrifugal pump having an outer case including a front wall, a backwall, and a peripheral outer case ring; a central rotatable shaftsupported in said case; shaft seal means sealing said shaft with respectto said case; an inner case including an annular, peripheral, inner casering, and a pair of opposite, end case plates fitted to the oppositesides of said inner peripheral case ring and defining a chambertherewithin; impeller means carried by the shaft in said chamber; volutemeans on said case plates and extending into and across said chamber inabutting engagement with one another to form a complete volute and in atleast partially surrounding relationship to said impeller; and forcebalancing partition means on said case plates extending into saidchamber in abutting engagement and in operative relationship with saidimpeller and said volute means to balance radial forces imposed on theimpeller generated by pressure at the outlet of said volute means. 19.An impeller for use in a centrifugal pump, said impeller formed of amaterial such as vinylidene fluoride or the like which is normallysubject to thermal shock when subjected to temperature change, aplurality of closed pumping ports on the front of the impeller extendingfrom adjacent the axis to the periphery thereof, a plurality of closedexpeller ports on the back of the impeller extending from adjacent theaxis to the periphery thereof, alternate cut-out areas on the front andback of said impeller between the pumping ports and between the expellerports, respectively, the walls of the impeller being of constantthickness between the front and back cut-out areas and around thepumping and expeller ports in a plane normal to the impeller axis and ina plane radial to the impeller axis, respectively, so as to reducethermal shock in the impeller.